1. Technical Field
The invention relates to an interferometric method and apparatus for measuring parameters of a device-under-test (DUT), particularly an optical element, for example an optical fiber, a sensor, a filter, a grating, a coupler, a power splitter, an interferometer, a modulator, a switch, integrated optics, and so on.
2. Background Art
The deployment of new optical technologies in optical networks is producing a demand for improved optical test and measurement techniques. As discussed in an article by Brian Soller entitled “Optical Vector Analysis Integrates Component Testing”, Lightwave Magazine, March 2004, for example, DWDM components increasingly must be capable of operating over narrow channel spacing with low loss and dispersion. A growing number of parameters of these components must be characterized carefully and accurately. Such components may require very precise optical alignment. Consequently, physical properties may vary with time, with a concomitant variation in optical properties. Moreover, they may have multiple input and output ports, and maybe tunable. To measure an increased range of parameters of such components efficiently and cost-effectively, whether during product development or production, imposes stringent demands upon the instrumentation.
In general, the parameters to be measured include insertion loss (IL), return loss (RL), group delay (GD), chromatic dispersion (CD), polarization-dependent loss (PDL), polarization-mode dispersion (PMD) and crosstalk/isolation. In order to allow all of them to be derived, the instrument must not only measure amplitude and phase as a function of frequency, but also as a function of polarization.
Traditionally, the above-described measurements were performed on separate test stations. Such an approach is time consuming and expensive as the component must be connected and reconnected multiple times as well as moved between test stations. It is also highly likely that redundant measurements would be taken.
To address the issues associated with using multiple test stations, various systems have been developed which integrate the multiple test stations on a single platform. Typically, such a system will include a tunable-laser source, a wavemeter, a polarization controller, a modulator, and optical receivers. Although such systems may overcome the problems associated with connecting and reconnecting the DUT and moving the DUT between test stations, multiple measurements must still be made, and the problem of time consumption remains.
The complete linear response of a DUT can be determined using a measurement method known as optical vector analysis (OVA). This method performs coherent interferometry (also referred to as swept-homodyne interferometry) for various states of polarization (SOP) and fully characterizes the component by measuring the attenuation (α(υ)) and phase shift (φ(υ)), as functions of optical frequency (υ) for each of various input SOPs and along different polarization axes at the output.
The transformation from input electrical field, {right arrow over (E)}0, to output electrical field, {right arrow over (E)}(υ), may be expressed as:{right arrow over (E)}(υ)=J(υ){right arrow over (E)}0  (1)where J(υ) is the Jones matrix. The four elements of the Jones matrix in a particular reference frame completely characterize the linear response of the DUT. Known OVA apparatus and methods determine the elements of the Jones matrix and, because they employ continuous scans, can provide both frequency domain and time domain characterization of the DUT. Hence, they can compute parameters in either domain.
It is known to perform OVA using a polarized source at the input of the interferometer and an analyzer/polarizer after the output of the interferometer, before the photodetector. Four measurements are performed, i.e., two orthogonal states of polarization are launched into the input of the interferometer, or input of the DUT, and, for each input state of polarization, the powers along two orthogonal polarization axes are measured at the output of the interferometer as a function of optical frequency The resulting four distinct curves are used to calculate the Jones matrix as a function of frequency. Examples of such instruments are described in U.S. Pat. No. 6,606,158 (Rosenfeldt et al.), U.S. Pat. No. 6,376,830 (Froggatt et al.), U.S. Pat. No. 6,788,419 (Cierullies et al.) and U.S. Pat. No. 6,813,028 (Vanwiggeren).
A limitation of these known instruments is that the two signals associated with the two input orthogonal states of polarization launched simultaneously into the DUT are detected, in essence, as modulations of two distinct carriers of different frequencies and so are distinguished separately in the frequency domain. Consequently, the bandwidth of the receiver front end, typically comprising the photodetector, amplifier and analog-to-digital converter (A/D), must be at least twice the bandwidth of each signal.
Such an increase in bandwidth of a photodetector results in an increase in the spectral density of the photodetector noise. Reducing the required bandwidth so as to maximise the signal-to-noise ratio (SNR) of the instrument, however, will reduce the maximum value of chromatic dispersion that can be measured and limit the maximum optical path length of the DUT. On the other hand, reducing the required bandwidth by decreasing the scan speed is also undesirable because it will lead to longer measurement times, thus exacerbating stability problems, and impairing measurement of chromatic dispersion. Moreover, commercially available tunable lasers tend not to operate smoothly enough at lower scan speeds, even if means are used that mitigate the effect of scan speed instability by applying a correction to the sampled signals.